Method of increasing the useful life of a molybdenum silicide heating element

ABSTRACT

A method of increasing the useful life of heating elements that consist essentially of molybdenum silicide and alloys thereof, wherein the heating elements operate at high temperatures in heat treatment processes and generally rest against the floor and/or the ceiling of a furnace. The heating elements contain aluminum to an extent sufficient to maintain a stable, slowly growing layer of aluminum oxide on the surfaces of the heating elements, and the heating elements are placed in direct abutment with an aluminum oxide brick material.

[0001] The present invention relates to a method of increasing theuseful length of life of heating elements when heat treating electronicceramics. Such ceramics may be ferrites (magnetic ceramics) andtitanates (e.g. BaTiO₃).

[0002] More specifically, the invention pertains to elements of thetypes molybdenum silicide and molybdenum tungsten silicide, includingdifferent alloys of these basic materials. Such elements aremanufactured by applicants in a number of designs.

[0003] Electronic ceramics are at present used in a number ofapplications in, e.g., optronics, mobile telephony and vehicleelectronics. Electronic ceramics that have a perovskite structure havedielectric, piezoelectric and ferroelectric properties, among otherthings. Examples of such materials include BaTiO₃ and Pb (Zr, Ti)0 ₃.Components that are typically manufactured from such materials areresonators, filters and capacitors. The capacitors are produced in theform of so-called multi layer capacitors (ML CC) for example. Theceramics are often brought into contact with a metallic conductor, suchas nickel, in the heat treatment stage.

[0004] When such heating elements are used in the manufacture ofso-called electronic ceramics, the elements are often placed on asupportive surface and heated respectively from the floor and ceiling ofcontinuous sintering furnaces. A long preferred design of heatingelements in such furnaces, for instance furnaces of the so-calledRiedhammer type for so-called ferrite sintering furnaces are so-called4-arm meander elements measuring {fraction (6/12)} mm on the heatingzone and connection part respectively.

[0005] Such elements rest typically on a supportive gravel layer ofaluminium-silicate particles, which, in turn, rests on a brickwork ofpurely aluminium oxide. The aluminum silicate, which is compatible (doesnot react chemically) with heating elements spectively. Sillimannite andmullite can be used for a long period of time before a reaction takesplace with the SiO₂-layer that develops on the surface of MoSi₂elements. This is because the aluminium silicate reacts more slowly withthe SiO₂ layer than the aluminium oxide brick. This reaction causesaluminium to be alloyed in the SiO₂-layer, therewith weakening theability of the layer to protect the element material; resulting in ashorter life length of the element.

[0006] Subsequent to this reaction with the surface layer a reactionalso takes place in the bulk material of the heating element, resultingin further corrosion and weakening.

[0007] In the absence of said particle layer, or gravel bed, a quickerreaction will take place by virtue of the SiO₂ on the surface of theelement being in direct contact with the Al₂O₃-brick.

[0008] Many electronic ceramics are heat treated at furnace temperaturesranging from 1200 to 1500° C. and higher. In addition to nitrogen gas,the atmosphere also typically contains about 5% hydrogen gas that has avarying dew point. For instance, the dew point may be+20° C.

[0009] It has been noted that in many cases the useful life length ofsaid elements falls significantly beneath the expected 3-5 year span,and in some cases only extends to a few months. The cause of local lifelength problems is the aggressive hydrogen gas that reacts with thegravel bed such as to form a smelt that contains chiefly aluminiumsilicate. The heating element sinks down in the smelt, wherewith anincrease in temperature takes place with accelerated corrosion andtemperature increase of the elements as a result. Moreover, the gravelbed sinters firmly into the aluminium oxide brick. The problem can alsooccur in atmospheres that do not contain hydrogen gas, wherewith theelement buries down into the particle layer as a result of temperaturechanges and therewith dimensional changes.

[0010] The gravel bed, or particle layer, also makes manufacture,transportation and furnace servicing more difficult to carry out.

[0011] It is therefore desired to eliminate the gravel be from theprocess.

[0012] The present invention fulfills this desideratum.

[0013] The present invention thus relates to a method of increasing theuseful life of heating elements that are comprised essentially ofmolybdenum disilicide and of alloys of this basic material, where saidelements operate at-high temperatures in heat treatment processes, andwhere said element rests on the floor and/or against the ceiling of afurnace, wherein the method is characterised by causing said heatingelements to lie in direct abutment with the aluminium oxide brick; andin that the heating element material contains molybdenum silicide andalloys thereof, wherein said material is caused to contain aluminium inan amount sufficient to maintain a stable, slowly growing layer ofaluminium oxide on the surface of respective heating elements.

[0014] The invention will now be described in more detail, partly withreference to the accompanying drawing, where

[0015]FIG. 1 illustrates a furnace design in which the invention isapplied.

[0016]FIG. 1 illustrates part of a furnace of the kind concerned. Thedrawing represents a segment, e.g. a heating zone, of a so-calledthrough-pushing furnace in which the material passes through the spacebetween the heating elements on, for instance, a ceramic tray that ispushed forwards on rails, for instance.

[0017] For heating the material from above, a gravel layer or bed ofparticles 1 is placed on an aluminium oxide plate 2. A four-arm element3 is placed on top of the particle bed. A “heating cassette” includingaluminium oxide brick-gravel bed-elements is pushed into acompartment/shelf plane 4 in surrounding brick insulation 5. The sameprocedure is used for elements that are heated from above and frombelow, with the difference that when heating from below there is used anupper aluminium oxide plate 6. Naturally, other furnace constructionscan be used that apply the principle in which the element rests on agravel bed.

[0018] The present invention relates to a method of increasing theuseful life of heating elements that consist essentially of molybdenumdisilicide and alloys of this basic material, where said elementsoperates in the heat treatment process at high temperatures in, e.g., acorrosive atmosphere that contains hydrogen gas, and where said elementsrest against the furnace floor and/or lie in abutment with the furnaceceiling.

[0019] In the case of the illustrated embodiment, the heating elements 3are in direct abutment with aluminum oxide brick. This means that thegravel bed 1 in FIG. 1 is excluded in accordance with the invention.

[0020] The material from which the heating element 3 is comprisedincludes molybdenum silicide and alloys thereof said material beingcaused to contain sufficient aluminium to maintain a stable, slowlygrowing layer of aluminium oxide on the surface of the heating element.

[0021] In the case of this embodiment there is thus obtained a slowlygrowing layer of aluminium oxide that is stable in the temperature rangerelevant in respect of the heat treatment of electronic ceramics.

[0022] According to one preferred embodiment, the material from whichthe heating element is comprised is caused to includeMo(Si_(1-x)Al_(x)), and is also caused to contain an aluminiumsufficiency.

[0023] According to one embodiment, x is in the order of 0.2-0.6.

[0024] It is preferred that x lies in the range of 0.40-0,50. Thisresults in a stable oxide layer while, at the same time, obtaining acomposition that is highly temperature durable and has good mechanicalproperties.

[0025] According to one preferred embodiment of the invention, theheating element material contains up to 40% by volume Al₂O₃. Thealuminum oxide constitutes an element mechanical stabilising phase andcounteracts abnormal particle size enlargement of the aluminosilicidephase.

[0026] The inventive method thus obviates the need for a gravel layer,therewith facilitating manufacture, transportation and furnaceservicing. The reason why a gravel layer is not required, is becausealuminium oxide lies against aluminium oxide.

[0027] Furthermore, the risk of corrosion of the system, heatingelement/gravel layer/aluminium oxide brick, is reduced, therewithgreatly extending the useful life of the heating elements and therebyminimising operational disturbances.

[0028] The suitability of the element material in the aforesaid contextis based on the inherent property of the Al₂O₃ ceramic to resistcorrosion in corrosive environments up to very high temperatures.

[0029] The present invention shall not be considered to be limited tothe described type of furnace, but can be applied with other types offurnaces where the aforesaid problems exist.

1. A method of increasing the useful life of heating elements thatconsist essentially of molybdenum silicide and alloys of this basicmaterial, said elements operating at high temperatures in heat treatmentprocesses and resting against the floor and/or the ceiling of a furnace,characterised by causing the heating element to lie in direct abutmentwith aluminium oxide brick material, wherein the heating elementmaterial contains molybdenum silicide and alloys thereof, and whereinsaid material is caused to contain aluminium to an extent sufficient tomaintain a stable, slowly growing layer of aluminium oxide on thesurface of the heating element
 2. A method according to claim 1,characterised in that the heating element material containsMo(Si_(1-x)Al_(x))₂, said material being caused to contain a sufficiencyof aluminium.
 3. A method according to claim 1 or 2, characterised inthat x lies in the range of 0.2-0.6.
 4. A method according to claim 1, 2or 3, characterised in that x lies in the range of 0.40-0.50.
 5. Amethod according to claim 1, 2, 3 or 4, characterised in that theheating element material contains up to 40% by volume Al₂O₃.